Method of operating vacuum induction furnace



Dec. 26, 1961 H. BUSSARD EIAL 3,014,255

METHOD OF OPERATING VACUUM INDUCTION FURNACE Filed Nov. 15, 195? INVENTORS AOR/[IV b. -8l/JJARD GOV/4R0 M421 [R A TTORNA'YJ United States Patent Ofiicc 3,014,255 Patented Dec. 26, 1961 3,014,255 METHQD (lF OPERA'HNG VACUUM INDUCTION FURNACE Adrien H. Bussard, Brnchkohel, near Hanan {Main}, and Gerhard Muller, Hanan (Main), Germany, assignors to W. C. Hcraeus G.m.b.H., Hanan (Main), Germany, a corporation of Germany Filed Nov. 15, 1957, Ser. No. 6%,756 2 Claims. (C1. 22-409) The present invention relates to new improvements in vacuum induction furnaces and to a new method of operating the same.

The most essential parts of an induction furnace are the crucible which is to be filled with the metal to be melted in the furnace and the induction coil which surrounds the cmcible and through which the high and lowfrequency currents flow for heating the metal by induction. If such an induction furnace is operated under a vacuum, it is very diflicult to tap the crucible and to cast the molten metal from it into any kind of molds. The vacuumsurrounding the crucible, that is, the vacuum tank which is fully closed toward the outside, does not permit any access to the crucible containing the molten metal and does not allow the metal to be tapped as easily from the outside as is the case in other types of smelting furnaces.

It has therefore been proposed prior to this invention to provide the crucible with an outlet channel which extends through the bottom of the crucible and, since it is heated less than the crucible itself, contains the metal in a solid form. This metal then serves as a plug for closing the outlet. In order to permit the molten metal to be tapped under a vacuum, this outlet channel is extended slightly beyond the bottom of the crucible and surrounded by a second induction coil. When the contents of the .crucible are melted suificiently, this second coil may be used to melt the solidified plug so that tr e molten metal may then flow out of the crucible and be cast into a mold.

A vacuum induction furnace of this type has the serious disadvantages that the melting process can be carried out only once and that the entirecontents of the crucible must be discharged therefrom and be cast into a mold in one lot.

It is the principal object of the present invention to provide a vacuum induction furnace which permits the contents of the crucible to be tapped several times to cast several portions thereof into different molds.

Another object of the invention is to provide a very simple mechanism which permits such repeated tapping to be carried out without any extensive control operations or manipulations from the outside, and which also permits the crucible to be refilled from the outside so as to'allow the furnace to be'operated continuously or semicontinuously.

'One'essential feature of the invention for attaining the above'objects consists in the provision of a movable casting plug in addition to the crucible as described above with a bottom outlet channel and a second induction-coil around this outlet.

The present invention'then permits the casting process to be carried out as follows:

At first, the-melting process iscarried out in the usual manner by melting the metal which has been filled into the furnace in pulverized or lump form, and by transferring the same into a homogeneous molten mass by heating it for a suiiicient length of time and with sufficient intensity. Thereafter, the induction coil is switched off and the crucible with themolten metal therein is allowed to remain in the furnace for a certain length of time without being furtherheated. During this'time, the

molten metal decreases from the high temperature necessary for mixing the contents of the crucible to a temperature which lies only slightly above the melting point of the molten metal. At this time, the coil around the outlet channel is switched on to 'melt the plug of solidilied metal within the channel and to allow the molten metal to flow into a mold underneath the taphole. However, a molten charge which has a temperature only slightly higher than the melting point would flow off very slowly and result in a poor ingot. Therefore, the second coil around the outlet channel will be supplied during the casting process with such an energy that the molten metal while passing through the outlet channel will again be heated up to the temperature required for a proper casting.

After one mold has thus been filled, the energy in the second coil is increased to such an extent that the stream of molten metal will whirl around in an eddy which narrows down the stream. The stream of molten metal and the amount of metal flowing through the taphole is then so small that it will be possible to move a shutter member which may consist, for example, of a ceramic material, underneath the tapholc and to close the opening completely. Thereafter, the second coil will be switched off and the subsequent cooling of the metal will again result in the formation of a plug of solidified metal within the outlet channel. In the meantime, the next mold is moved underneath the taphole, the shutter member moved away from the taphole, and the second coil is switched on so that the next casting operation can proceed. This procedure may be repeated until the crucible is either entirely empty or all the molds which are held ready within the vacuum chamber are filled. The subsequent interruption of the casting operation also permits the crucible to be refilled with a new charge of metal in a pulverized or lump form by means of a gate mechanism without requiring the vacuum in the main vacuum chamber to be released.

This new casting method maythereforebe carried out entirely continuously or at least semicontinuously.

Further objects, features, and advantages of the present invention will be apparent from the following detailed description thereof, particularly when read with reference to the accompanying drawing which is merely intended to illustrate the new method and to indicate diagrammatically one preferred embodiment of a vacuum tank in which the method may be carried out.

Referring to the drawing, the vacuum chamber 1 which is used for the actual casting process is enclosed by a vacuum tank 2 withan inner wall 3 and an. outer cooling jacket 4, and a removable cover 5 which is likewise enclosed by a cooling jacket 6. The water inlets and outlets and the direction of flow of the cooling water are indicated at '7. This tank 2 contains the crucible 8 which is adapted to hold the charge of molten metal 9 and is surrounded by the main induction coil 10. The bottom of crucible 8 is extended downwardly to form a neck 11 with an outlet channel 12 therein which terminates in a taphole 13. Neck 11 is surround by a second induction coil 14 which is used for melting a plug of solidified metal within outlet passage 11 and for reheating the molten metal within the passage. Underneath taphole 13 is a shutter member 15 which may, for example, be pivotable horizontally, as shown in the drawing, for closing and opening the taphole during the inter-missions between the individual casting operations. This shutter member .15 may consist, for example, of ceramic material. Underheath the actual melting chamber 1 is a second chamber 16 which is adapted to receivea plurality of molds 17 which may be mounted on a suitable conveyor, for .example, a carriage 18. This carriage is movable on a track 19 by a suitable control mechanism operable from the outside to move one mold 17 intermittently after the other to a point directly underneath ta-phole 13. If desired, a casting funnel may be mounted at a point between taphole 13 and molds 1'7 and coaxialiy with outlet channel 12. Chamber 16 has at least at one side a gate 21 which may be opened or closed airtight and which connects chamber 16 with an antechamber 22 which is provided with another gate 23 leading to the outside, and a suction pipe 24 which is connected to a vacuum pump, that is, either to the same pump which is used for evacuating the main vacuum chambers 1 and in or to a difierent pump, for evacuating chamber 22 separately from chambers 1 and 16 when gates 21 and 23 are closed airtight. Thus, after one carload of molds 17 has been filled within chamber 16, it may be moved aside and, after chamber 22 has been evacuated, gate 21 may be opened to move the next carload of molds 17 into chamber 16 to be filled.

In order to permit a continuous operation, another vacuum chamber 25 is preferably provided at the other side of the casting chamber 16. This chamber likewise has a gate 26 connecting it with chamber 16, another gate 27 leading to the outside, and a suction pipe 23 which is connected to a suction pump. After one complete car-load of molds 17 has been filled in chamber 16 and after chamber 25 has been evacuated, gate 26 may be opened and molds 17 may be moved into this delivery chamber 25. At the same time, a new supply of empty molds 17 may be moved from chamber 22 into chamber 16. Thereafter, gates 21 and 26 may again be closed and gates 23 and 27 be opened to remove the filled molds 17 from delivery chamber 26 and to roll another carload of empty molds into the antechamber 22. Consequently, the vacuum in the main chambers 1 and 16 never needs to be released during the entire operation.

Instead of requiring the removal of the entire cover 5 from vacuum tank 2 for filling and refilling crucible 9 which would also require a release and subsequent reestablishment of the vacuum in chambers 1 and 16, cover 5 is provided in a central position with a hopper 29 which is likewise provided with a pair of gates or covers 39 and 31. Gate 30 is disposed on the inside of chamber 1 and may be opened and closed from the outside by suitable means, not shown, While cover 31 may be opened from the outside to fill it with a new charge of metal in a pulverized or lump form which, after cover 31 has again been closed, and after hopper 2 has also been evacuated through a pipe 32 and cover 30 has been opened, is dropped into crucible 8.

As already indicated, all of the vacuum chambers, including the main chambers 1 and 16, the auxiliary chambers 22 and 25, and hopper 29, may be evacuated by a single strong pump unit which is provided with suitable control means for connecting and disconnecting the individual chambers thereto and for regulating the vacuum within these chambers. Preferably, however, at least the large main chambers 1 and 16 are connected through a suction pipe 33 of adequate cross-sectional area to a separate suction pump, preferably a Roots-type pump 34, of such strength as to insure a proper evacuation of chambers 1 and 16 even though the amount of gases developing during the melting process and escaping from crucible 8 might be very considerable.

Summarizing the above, the casting process proceeds as follows:

First, with gates 21 and 26 and cover 30 being closed, the main chambers 1 and 16 are evacuated through pipe 33. At the same time, hopper 29 is filled with metal in a pulverized or lump form, whereupon cover 31 is closed and hopper 29 evacuated through pipe 32.. When such evacuation has been carried out sufficiently, cover 30 is opened and the metal dropped into crucible 8. Then, the main induction coil 19 is switched on and the metal in crucible 3 is melted so that the crucible will then be partly filled with molten metal. During this melting process, the metal passes from the solid to a highly viscous liquid state, sometimes referred to as a slurry. A small part of this slurry runs into nozzle 11 and since the coil 14 is not energized at this time, the slurry running into the nozzle will resolidify within the channel 12 and close the outlet opening or t phole 13.

During this first melting process, carriage 13 with the empty molds 17 thereon is rolled into antcchambcr 22 which is then closed and evacuated through pipe 24. When the vacuum in antechamber 22 is substantially equal to that in the main chambers 1 and 16, gate 21 will be opened and carriage 18 wheeled into the casting chamber 16 to a point where the first of molds 17 will be directly underneath taphole 13. After the melting process in crucible 8 has been completed to such an extent that the molten metal 9 is thoroughly intermixed, the main induction coil 10 is switched off and the metal allowed to stand for a certain length of time until it has cooled off to a temperature only slightly higher than the solidification point. Then, induction coil 14 is switched on to melt the solid metal plug in outlet passage 12. The molten metal then flows from taphole 13 into the first mold 17 underneath. During this casting process, coil 14 remains switched on and then serves to heat up the metal during its flow through outlet channel 12. to such a temperature that an ingot which is almost pipeless will be cast in mold 17. When this mold is almost completely filled, the energy in coil 14- will be considerably increased by suitable control means with the result that, through the eddy currents then forming in the molten metal in outlet channel 12, the stream of metal passing through taphole 13 will be narrowed down considerably.

At this time, the shutter member 15 is pivoted from its lower position 15' so as to cover taphole 13, whereupon coil 14 is switched off. The cold shutter member 15 together with the subsequent cooling of the metal in outlet channel 12 results in a very quick solidification of the metal within the outlet so that another solid metal plug will form therein. In the meantime, carriage 13 has been moved forwardly, that is, toward the left as seen in the drawing, to such an extent that the next empty mold 17 will be disposed accurately underneath taphole 13. Shutter member 15 is then pivoted away from taphole 13 and coil 14 is again switched on so that the metal plug in channel 12 will melt and the molten metal can again flow into the next mold underneath. This procedure is repeated until almost the entire molten metal 9 contained in crucible 8 has been cast into molds 17.

After the last remainder of molten metal 9 in crucible 8 has again formed into a solid metal plug in channel 12 in the same manner as previously described, cover 30 of hopper 29 which has previously been evacuated will be opened and a new charge of solid metal in a pulverized or lump form be supplied into crucible 8, whereupon cover 30 is again closed and the entire procedure as above described may be repeated as often as desired starting with the step of switching on the main induction coil 10 to melt the new charge of metal in crucible 8.

Since the removal of the filled molds 17 from chamber 16 and the feeding of a new supply of empty molds into this chamber may be carried out by the evacuation of the auxiliary chambers 22 and 25 and the operation of gates 21, 23, 26, and 27 without requiring the vacuum in the main chambers 1 and 16 to be released, and since the operation of refilling the crucible may also be carried out without any release of the vacuum in these main chambers, all of the time which it would take to re-evacuate these large chambers will be saved, the periods between the individual casting operations into the succession of empty molds will be very short, and the entire operation may be carried out continuously or at least semicontinuously. The efiiciency and rate of production of the new apparatus is therefore far superior to any similar apparatus as previously known.

Although our invention has been illustrated and described with reference to the preferred embodiments thereof, we wish to have it understood that it is in no way limited'to the details of such embodiments, but is capable of numerous modifications within the scope of the appended claims.

Having thus fully disclosed our invention, what we claim is:

1. A method of operating a vacuum induction furnace comprising the steps of melting a charge of metal under a vacuum in a crucible by passing an electric current through an induction coil around the main upper part of the crucible, allowing the molten metal to cool in a lower outlet channel of the crucible to form a plug of solid metal to close the outlet opening, passing an electric current through a second induction coil around the outlet channel to melt the plug and to start the flow of metal through the outlet opening, increasing the current passing through the second induction coil to heat the molten metal flowing through the outlet channel to the proper temperature for casting, temporarily increasing the current passing through said second induction coil during the casting operation for heating the stream of metal flowing through the outlet channel to such an extent as to whirl the metal in an eddy therein to reduce the volume of said stream, moving the closure member against the outlet opening to shut off the flow of metal therefrom, de-

energizing the second induction coil to allow the molten metal in the outlet channel to cool and form a plug of solid metal closing the outlet channel and, removing the closure member from the outlet opening.

2. In a method of operating a vacuum induction furnace having an outlet channel with an outlet opening in the lower end thereof, a first induction coil surrounding the upper part of the crucible, a second induction coil surrounding the outlet channel and a closure member adapted to close the outlet opening, the method which comprises placing a charge of metal in the crucible, melting the charge of metal in the crucible by passing an electric current through the first induction coil, allowing the molten metal to cool in the outlet channel to form a plug of solid metal to close the outlet opening, tie-energizing the first induction coil and allowing the molten metal in the crucible to cool to a temperature slighly above the melting point, passing an electric current through the second induction coil to melt the plug and to start the flow of metal through the outlet opening, increasing the current passing through the second induction coil to heat the molten metal flowing through the outlet channel to the proper temperature for casting, temporarily increasing the current passing through the second induction coil to form an eddy in the stream of metal flowing through the outlet channel to reduce the volume of said stream, moving the closure member against the outlet opening to shut off the flow of metal therefrom, de-energizing the second induction coil to allow the molten metal in the outlet channel to cool and form a plug of solid metal closing the outlet channel and removing the closure memher from the outlet opening.

References Cited in the file of this patent UNITED STATES PATENTS 611,142 Pignotti et a1, Sept. 20, 1898 1,803,135 Ross Apr. 28, 1931 1,830,686 Woods Nov. 8, 1931 2,085,450 Rohn June 29, 1937 2,336,518 Whittaker Dec. 14, 1943 2,686,864 Wroughton et al Aug. 17, 1954 2,686,865 Kelly Aug. 17, 1954 2,707,720 Tama May 3, 1955 2,743,492 Easton May 1, 1956 2,754,346 Williams July 10, 1956 2,796,644 Kuhn June 25, 1957 2,837,790 Rozian June 10, 1958 2,882,570 Brennan Apr. 21, 1959 

